As technology progresses, wireless devices tend to integrate more features and become increasingly ubiquitous. For example, wireless devices may include features associated with personal digital assistants (PDAs), cellular telephones, wireless internet access devices, global positioning system (GPS) receivers, and the like. Therefore, multiple radio frequency (RF) signals spanning a wide frequency range are common in many areas and may interfere with one another. Wireless devices may support one or more wireless communications protocols, such as third (3G), fourth (4G), or later generation cellular telephone protocols, GPS protocols, wireless fidelity (Wi-Fi) protocols, Bluetooth®, and the like. Some of these protocols may have tight channel spacings that require narrow channel bandwidths. Therefore, such wireless devices using narrow channel bandwidths may need RF filters that have narrow passbands to accept desirable signals and have maximum broadband rejection to remove interfering signals.
Surface acoustic wave (SAW) devices are electro-mechanical devices that utilize surface acoustic waves, which travel along the surface of a piezoelectric material having some elasticity. Piezoelectric material has a relationship between an electrical signal and mechanical movement within the material. An applied electrical signal may be used to cause the mechanical movement, which may include surface acoustic waves, or the mechanical movement may be used to provide an electrical signal. Therefore, the piezoelectric material may be used as a transducer for transduction between an electrical signal and mechanical movement. An inter-digital transducer (IDT) is a special type of transducer used to convert an electrical signal into surface acoustic waves or to convert surface acoustic waves into an electrical signal.
A SAW filter may use two IDTs. A first IDT may be acoustically coupled to a second IDT. The first IDT receives an input signal and the second IDT provides an output signal based on the surface acoustic waves that are coupled between the IDTs. Such a SAW filter may provide a narrow passband. Rejection of signals outside the passband depend upon impedances between the IDTs and impedances between each IDT and ground. These impedances may be affected by construction and layout of the SAW filter and parasitic characteristics of the SAW filter. Typically, SAW devices that are used in RF communications equipment may require low impedances needed by such equipment. Additionally, to preserve signal integrity, SAW devices may need to have minimal insertion loss. Thus, there is a need for a SAW device having low impedance, minimal insertion loss, a narrow passband, and maximum broadband rejection.